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Thread: Evohome calling for heat when off

  1. #1
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    Default Evohome calling for heat when off

    I've had an evohome system installed for over a year. It has 6 HR92s.

    The problem I have is that the system calls for the boiler to be on after about 5-10 minutes after I've turned the heating off by using the quick action on the controller, or the heating is off by timer.

    When I check the system summary it says Boiler 100% but everything else is 0%, so why is the boiler on?

    Thanks
    Anthony

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    Hi Anthony - not sure of your fuel source/ boiler cycles per hour etc but do you think this may be just the boiler dissipating heat through a bypass via pump overrun especially when all 6 valves close in quick succession?

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    Quote Originally Posted by demusss View Post
    I've had an evohome system installed for over a year. It has 6 HR92s.

    The problem I have is that the system calls for the boiler to be on after about 5-10 minutes after I've turned the heating off by using the quick action on the controller, or the heating is off by timer.
    Up to about 5 minutes delay when using heating off is normal behaviour. There is a delay of up to around 4 1/2 minutes from when the controller sends set point changes out to the HR92's until they close their valves and request zero heat. 10 minutes would be a little bit out of the ordinary though.
    When I check the system summary it says Boiler 100% but everything else is 0%, so why is the boiler on?
    That doesn't seem possible. The Boiler heat demand should be equal to the highest heat demand from any of the zones. Are you sure there aren't some zones on a second page and you didn't notice ? If you have more than a handful of zones they won't all fit on the first page on the diagnostics page.

    If you really do see boiler relay 100% and all zones are 0% I would try rebooting the controller.

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    Automated Home Legend paulockenden's Avatar
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    I've seen that - all zones reporting 0% but the boiler demand with a higher percentage. It can sometimes take a minute or so for the controller to realise it should be sending 0% to the boiler. Maybe it's to do with the cycles set up, even though I'm using Opentherm so that really shouldn't matter.

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    I'll try to take a photo next time, normally its when I'm in bed when I notice.

    When I have checked the green light on the Evo Boiler control is green, so its definitely the controller calling for heat. I hear the TRVs close and then later on the boiler fires up.

    Boiler is due to be changed very soon, hopefully to a Opentherm compatible one so I'll get the Opentherm bridge to go with it, maybe that will sort it out.

    How would I go about rebooting the controller?

    Thanks all for the advice

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    Well it happened again last night and I managed to take photos.

    Heating was set to off using the quick action on the controller
    Bedroom TRV was manually set to 22 degrees using the TRV knob
    Before going to sleep I used my mobile to turn things off (why get out of bed when you don't have to)
    did this by opening up app
    cancelled quick action on mobile
    changed bedroom temperature to 5 degrees
    turned heating off using quick action

    I didn't hear the TRV close after doing all this so I checked the system summary on the controller.
    The TRV and boiler were both set at 8%. The main screen on the controller was showing the heating was off.
    So I turned the TRV knob and set the TRV to 5 degrees.
    The system summary now showed 0% on the TRV and boiler.

    After about 5 minutes I heard the boiler fire up. Checked the system summary and everything was still 0%. Checked the receiver at the boiler and the green light was on. Checked the main screen and everything was showing as off. The boiler stayed on for only about a minute.

    I took photos showing system summary with everything at 0%, then the boiler receiver showing the green light on, then the main screen showing everything should be off.

    So why does the boiler get turned on?

    Thanks
    Anthony
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  7. #7
    Automated Home Legend paulockenden's Avatar
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    That's different to the example I've seen, where all the zones are zero percent but the controller still shows a boiler demand.

    In your example the controller doesn't turn the boiler on and off. It sends a duty cycle (so many minutes on, so many minutes off) from time to time, with a frequency as defined in the system parameters. It's daft that a zero boiler demand doesn't get immediately sent to the boiler control, instead it waits for the next cycle. But that's the way it seems to be programmed.

    P.

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    Quote Originally Posted by demusss View Post
    Well it happened again last night and I managed to take photos.

    Heating was set to off using the quick action on the controller
    Bedroom TRV was manually set to 22 degrees using the TRV knob
    Before going to sleep I used my mobile to turn things off (why get out of bed when you don't have to)
    did this by opening up app
    cancelled quick action on mobile
    changed bedroom temperature to 5 degrees
    turned heating off using quick action

    I didn't hear the TRV close after doing all this so I checked the system summary on the controller.
    How long did you wait ? It will take up to 4 1/2 minutes for the HR92 to adjust after using a phone app override....

    Also the phone app itself (and/or the web API it calls) are not super reliable, so sometimes requested changes will fail to take place. Sometimes you'll get an email alert a few minutes later to warn you of this (do you have email alerts enabled in the Honeywell app ?) but sometimes you won't.

    I would have gone to have a look at the controller screen to see if the set point had updated there or not. Normally a set point change made on the phone app should be reflected on the controller screen within about 10 seconds, but there is an additional delay in that being sent to the HR92.
    The TRV and boiler were both set at 8%. The main screen on the controller was showing the heating was off.
    So I turned the TRV knob and set the TRV to 5 degrees.
    The system summary now showed 0% on the TRV and boiler.

    After about 5 minutes I heard the boiler fire up. Checked the system summary and everything was still 0%. Checked the receiver at the boiler and the green light was on. Checked the main screen and everything was showing as off. The boiler stayed on for only about a minute.

    I took photos showing system summary with everything at 0%, then the boiler receiver showing the green light on, then the main screen showing everything should be off.

    So why does the boiler get turned on?
    From your screenshots and description, in this particular instance there will have been a lost comms message between the controller and the boiler relay. The controller tried to send a 0% heat demand to the boiler relay but it didn't receive it for whatever reason. Could be numerous reasons - collision with another 868Mhz device transmitting at the same time, (another honeywell device or other devices) noise interference, or marginal signal.

    The Boiler relay continues at its previously requested duty cycle until it either receives a new heat demand or hasn't heard from the controller for 45 minutes - if that happens the red light will flash as a warning of extended comms failure. If it doesn't hear from the controller for 60 minutes the red light will go on solid and the boiler relay will go into failsafe mode if you have that enabled in your system settings.

    The heat demand from the controller to boiler relay is re-transmitted every 20 minutes even if the heat demand is not varying so assuming the following transmission got through without interference the boiler would then go fully off at that time.

    Key to understanding these kind of problems is that the controller does not directly command every on/off switching of the relay (except for the hot water relay where it does) it only requests heat demands as a percentage, and its then up to the relay itself to follow a regular (default 10 minute) on/off cycle to meet that heat demand. In the absence of further information from the controller it just keeps following that previously requested cycle - at least up until 60 minutes when it will finally give up and assume a complete loss of comms.
    Last edited by DBMandrake; 29th October 2019 at 10:40 AM.

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    Quote Originally Posted by paulockenden View Post
    That's different to the example I've seen, where all the zones are zero percent but the controller still shows a boiler demand.
    I've seen both scenarios - it just depends on which comms messages were lost - heat demand from HR92's to controller, or heat demand from controller to BDR91.
    In your example the controller doesn't turn the boiler on and off. It sends a duty cycle (so many minutes on, so many minutes off) from time to time, with a frequency as defined in the system parameters. It's daft that a zero boiler demand doesn't get immediately sent to the boiler control, instead it waits for the next cycle. But that's the way it seems to be programmed.
    I'm not quite sure what you mean by that.

    If a zero heat demand is sent to a BDR91 it will go off immediately no matter where it is in it's (default) 10 minute cycle. That's how the hot water relay works - the controller just sends a 100% demand to turn it on and a 0% demand to turn it off, and if you've ever fiddled with the hot water schedule you'll notice that the hot water relay will go on and off immediately if you toggle hot water mode on and off with a cold enough cylinder.

    When it changes the heat demand from one partial demand to another partial demand you are correct - you can sometimes end up waiting a full cycle for anything to happen depending on what the change in heat demand is and where in the cycle it was when the change was requested.

    The way it seems to work is in each cycle the off period of the duty cycle is at the start of the cycle and the on period is at the end of the cycle. So a 40% heat demand would result in 6 minutes off at the start of the cycle and 4 minutes on. When the heat demand changes the switchover point just moves in the cycle, but the timer counting how far you are in the cycle just keeps moving as normal. So the movement of the switchover point could result in the relay switching immediately (for example if you were 50% through the cycle and heat demand went from 30% to 70% or vica versa) or switching sooner or later than it otherwise would have, but often in the following cycle.

  10. #10
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    Quote Originally Posted by DBMandrake View Post
    I've seen both scenarios - it just depends on which comms messages were lost - heat demand from HR92's to controller, or heat demand from controller to BDR91.
    I'm not sure how this can be lost comms. The controller itself shows all zones at 0% but (last night) boiler demand of 11%. If it was lost comms HR92->controller then the zones wouldn't all show 0%, and if it was lost comms controller->boiler then the boiler demand would show 0%.


    I'm not quite sure what you mean by that.
    What I meant - sorry if it wasn't obvious, I thought it was - was that during partial demand the controller isn't responsible for pulse-firing the boiler. That logic is local to the DBR91 (or whatever). I thought from the message I'd replied to that the poster was under the impression that the controller sent the individual switching messages for the boiler cycling.

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